If a char array is an Object in Java, why does pri

问题:

Printing a char array does not display a hash code:

class IntChararrayTest{
    public static void main(String[] args){
        int intArray[] = {0,1,2};
        char charArray[] = {'a','b','c'};
        System.out.println(intArray);
        System.out.println(charArray);
    }
}

output :

[I@19e0bfd
abc

Why is the integer array printed as a hashcode and not the char array?

回答1:

First of all, a char array is an Object in Java just like any other type of array. It is just printed differently.

PrintStream (which is the type of the System.out instance) has a special version of println for character arrays - public void println(char x[]) - so it doesn't have to call toString for that array. It eventually calls public void write(char cbuf[], int off, int len), which writes the characters of the array to the output stream.

That's why calling println for a char[] behaves differently than calling it for other types of arrays. For other array types, the public void println(Object x) overload is chosen, which calls String.valueOf(x), which calls x.toString(), which returns something like [I@19e0bfd for int arrays.

回答2:

The int array is an array of integers where as the char array of printable characters. The printwriter has the capability to print character arrays as this is how it prints string anyway. The printwriter will therefore print them like a string, without calling the toString() method to convert it to a string. Converting an int array to a string returns a hash code, explaining why you get that output.

Take this for example:

int[] ints = new int[] { 1, 2, 3 };
char[] chars = new char[] { '1', '2', '3' }

If you were to print both those sequences using the method you used, it would print the hash code of the int array followed by '123'.

回答3:

System.out is PrintStream which has a special method for char[] arg

public void println(char x[]) {
    synchronized (this) {
        print(x);
        newLine();
    }
}

int[] is printed via this method

public void println(Object x) {
    String s = String.valueOf(x);
    synchronized (this) {
        print(s);
        newLine();
    }
}

回答4:

Ofcourse arrays are objects in Java. No matter what type of array it is. It is always an object in Java.

But as far as your question is concerned, there is a method, println(char[] array), in java.io.PrintStream class that prints out all characters from a char array. And that is the reason, when you pass a char[] as a parameter to println(), it doesn't call the toString() method of the java.util.Array class, but rather loops through the array and prints every character.

Citation:

http://docs.oracle.com/javase/7/docs/api/java/io/PrintStream.html#println(char[])

回答5:

System.out actually gives you an Object of PrintStream. and its println(params) has some overloaded methods which are implemented differently for different type of params.

This params if being a char[] provides the output as char[] elements like System.out.println(charArray); outputs abc but not the hashcode as in the case of int[] like System.out.println(intArray); outputs [I@19e0bfd.

PS :- All the array are treated as Object in Java.

See the Official Doc

Moreover, for printing array, always use the Arrays utility class. Its method Arrays.toString() should be preferably used for printing array objects.

回答6:

The other answers correctly explain your case of a separate call to PrintStream.println and the same is true for .print. However your title could cover other ways of printing. PrintWriter.println and .print AND .write do have char[] overloads for a simple call.

However, formatting does not have a special case:

 System.out.format ("%s%n", charArray); // or printf
 myPrintWriter.format ("%s%n", charArray); // or printf

each outputs [C@hash, similar to the handing of int and other type arrays.

Nor does concatenation, often used in printing:

 System.out.println ("the value of charArray is " + charArray);

outputs the value of charArray is [C@hash.